Device and method for monitoring an electrolytic process

ABSTRACT

In manufacturing integrated circuits voids in the metal layer may readily form during electrolytic metal deposition. In order to avoid these faults which adversely affect the functionality of the circuits, the invention suggests to utilize for metal deposition an electrolysis device comprised of at least one anode and at least one cathode and in which at least one reference electrode is disposed at the surface of the at least one anode or at the surface of the at least one cathode. A voltmeter is respectively provided for detecting the electric voltages between the at least one anode and the at least one reference electrode and between the at least one reference electrode and the at least one cathode.

The invention relates to a device and a method for monitoring anelectrolytic process, more specifically for electrolytically depositingmetal onto a substrate.

Integrated circuits on wafers, more specifically made from silicon, aremanufactured generally using etching and deposition processes inconjunction with photolithographic processes. Up to the present,metallic conductive patterns have been customarily produced usingsputter processes for establishing electrical conductor connections onthe wafers. For some years galvanic processes have been increasinglyutilized to manufacture integrated circuits on wafers. Aside from theelectrolytic deposition of copper in what is termed the “back end”portion, i.e., for wiring the semiconductor structures produced on thewafer, the metal deposition of copper, nickel, gold and tin in what istermed the “packaging” process i.e., when metal is deposited onto chipcarriers, and in rewiring becomes increasingly important. All theserequirements have in common that an electrolytic metal depositionprocess is initiated on a thin starting metal layer, the so-called seedlayer, also termed plating base. For this purpose, the starting layer isplaced in electric contact using suited mechanical contacts and isplaced in an electroplating bath containing the metal to be deposited insolution. Current is caused to flow through the starting layer and thecounter electrode using an external current source, for example of arectifier energized by the electric network, and a counter electrode sothat metal is deposited onto the starting layer of the wafer. The amountand, as a result thereof, the coating thickness of the deposited metalis controlled through Faraday's law.

Coating thickness distribution on the wafer may be positively affectedusing suited shields or segmented anodes (counter electrodes). Solubleanodes are thereby used to keep the metal ion concentration in theelectrolyte fluid constant or, alternatively, insoluble (inert) anodes,in which case the metal ion content must be kept constant, makingadditional provisions.

U.S. Pat. No. 5,234,572 A describes a method for replenishing metal ionsto a plating bath. To control the quantity of electricity transmittedbetween cathode and anode when current is applied, U.S. Pat. No.5,234,572 A suggests a measuring arrangement that measures the potentialof the cathode (counter electrode) using a reference electrodeadditionally placed in the plating bath and made of the same metal asthe anode. The quantity of electricity conducted is controlled such thatthe measured potential of the counter electrode may not be negative withrespect to said reference electrode. This prevents metal ions fromdepositing on the counter electrode. In a preferred exemplaryembodiment, the counter electrode and a soluble electrode (anode) areconnected to a DC supply. A voltmeter is used for measuring thepotential of the counter electrode relative to the reference electrode.The potentials at the counter electrode and at the soluble electrode,which vary with the current flow, are shown in a figure.

In case of electrolytic copper depostion, DE 199 15 146 C1 mentions thatthe copper depostion bath contains, in addition to the usual bathconstituents, Fe(III) compounds for example and that these compoundscause the copper pieces to dissolve to form copper ions, yielding Fe(II)compounds in the process. The Fe(II) compounds formed are reoxidized toFe(III) compounds at the insoluble anodes.

All of the known electroplating baths contain, in addition to the metalions to be deposited, auxiliary agents for influencing the metaldeposition. As a rule, these agents are organic compounds forinfluencing the structure of the deposited layer on the one side andsalts, acids or bases that are added for the purpose of stabilizing thebath and of increasing the electric conductivity thereof on the otherside. The voltage required for deposition is reduced as a resultthereof, thus generating a minimum of Joule heat. This increases thesafety of the process. Certain processes are only made possible byadding these agents.

Generally, conductive patterns are presently produced according to thedamascene process. As recited in DE 199 15 146 C1, a dielectric layer isfirst applied on the semiconductor substrate for this purpose. The viasand trenches required for receiving the desired conductive pattern areetched in the dielectric layer, usually using a dry etch process. Aftera diffusion barrier (in most cases tantalum nitride, tantalum) and aconductive layer (in most cases sputtered copper) are applied, thedepressions, i.e., the vias and trenches, are electrolytically filledusing the “trench filling process”. As copper is deposited on the entiresurface, the excess must be subsequently removed from the unwantedlocations, meaning from the zones outside the vias and trenches. This isachieved using what is termed the CMP process (Chemical MechanicalPolishing). Multilayer circuits can be made by reiterating the processi.e., by repeatedly applying the dielectric, made from silicon dioxidefor example, forming the depressions by etching and depositing copper.

After the conductive pattern, more specifically made from copper, ismade, it can be found out, in the polished sections intended forcontrol, that metal defects (voids) readily form in the depositedstructures, said defects possibly leading to a functional breakdown ofthe entire circuit.

Therefore the problem the present invention is facing is to avoid thedrawbacks of the known devices and methods and more specifically to findmeasures permitting to reliably prevent such defects from forming.

In overcoming this problem the invention provides the device accordingto claim 1 and the method according to claim 7. Preferred embodiments ofthe invention are indicated in the subordinate claims.

The device according to the invention serves to monitor an electrolyticprocess, more specifically the electrolytic metal deposition process,during manufacturing of integrated circuits of semiconductor substrates(wafers) and of circuit structures on chip carriers.

To explain the invention in closer detail, the term “wafer” will be usedherein after to designate any workpiece. Likewise, the terms “depositionelectrolyte” or “deposition electrolyte fluid” will be used to refer tothe electrolyte fluid used for carrying out the electrolytic process.Alternatively, said fluid could also be an etch fluid if theelectrolytic process is an electrolytic etch process. Possibleelectrolytic processes are both electrolytic deposition methods andelectrolytic metal etching methods. In principle, the invention can alsobe utilized for other electrolytic processes than those mentionedherein. In the following description, the term “electrolytic depositionprocess” will be used to refer to all the other electrolytic processesas well.

In overcoming the problem, the invention provides a device and a method.The device is comprised of at least one anode and of at least onecathode that are in contact with an electrolyte fluid, an electriccurrent flow being generated between them. At least one referenceelectrode is disposed at (near) the surface of the at least one anode orat (near) the surface of the at least one cathode. A voltmeter fordetermining the respective electric voltages between the at least oneanode and the at least one reference electrode and between the at leastone reference electrode and the at least one cathode is further providedin accordance with the invention. This arrangement permits tosimultaneously register electrolytic partial processes at the variouselectrodes, this very provision permitting to also measure time-variantprocesses. It thereby makes no matter whether the electrodes are onlyimmersed into the electrolyte fluid during measurement or whether avoltage is applied between cathode and anode only when the twoelectrodes are contacting the electrolyte fluid.

In a preferred application of the device in accordance with theinvention, the cathode is a wafer or a chip carrier substrate and theanode a metal plate. In this case, preferably metal is deposited ontothe wafer or the chip carrier substrate during the electrolytic process.

The device in accordance with the invention is more specificallycomprised of at least one first reference electrode which is disposed at(near) the surface of the at least one anode, and of at least one secondreference electrode which is disposed at (near) the surface of the atleast one cathode. Voltmeters for measuring the electric voltagesbetween the at least one anode and the at least one first referenceelectrode, between the at least one first and the at least one secondreference electrode and between the at least one second referenceelectrode and the at least one cathode are further provided. Thismonitoring device serves to measure the electric voltages between theanode and the first reference electrode, between the first referenceelectrode and the second reference electrode and between the secondreference electrode and the cathode.

Comprehensive tests showed that the defects (e.g. voids) in thedeposited metal layers are due to the fact that the metal depositionbaths used are capable, under certain conditions, of removing metal fromthe starting layers:

When a wafer provided with a starting metal layer is immersed in themetal deposition solution, no external voltage is applied to thestarting layer at first. Accordingly, an equilibrium potential isachieved at the phase boundary between the starting layer and theelectrolyte solution as soon as the two are coming into contact. Underthe usual conditions applied for depositing metal, more specificallycopper, onto the starting layer, the potential of the starting layerrelative to the metal dissolution is positive so that said startinglayer slowly dissolves in the deposition solution.

Metal starting layers which are utilized on wafers are generally verythin for reasons related to cost and process. For example, in typicalstructures (trenches, vias of e.g., 0.1-0.2 μm wide and about 1 μm deep)made for the damascene process, the starting layers usually are fromabout 5-25 nm thick. By contrast, the starting layers on the surfaces ofthe wafers are about 100 nm thick. Such type layers can be quicklyremoved, at least within the structures, during immersion in theelectrolyte solution since the etch rate in the electrolyte solutionsused is quite high. In a typical copper electrolyte containing about 180g/l sulfuric acid and 40 g/l copper in the form of copper sulfate, theetch rate is about 10 nm/min under usual electrolysis conditions. Underthese conditions, the coating thickness that remains prior to metaldeposition may, under certain circumstances, not suffice to ensurereliable metal coating. The etch rate depends, inter alia, on the typeof electrolyte solution used, the conditions chosen for the depositionprocess and the type of starting layer.

This problem cannot be overcome by shortening the time between immersionand the start of the deposition process since a certain minimumprocessing time must be observed after immersion in order for example tocompletely wet the wafer to be coated with fluid prior to starting metaldeposition. Accordingly, the time window available for the process ofelectrolytically depositing metal onto the starting layer is but anarrow one. A particular problem is that, due to the plurality ofpossible influencing variables, the size of the time window for theprocess cannot be determined so that the result of metallization is leftto chance only.

The thin starting metal layer is particularly sensitive to fluctuationsof the deposition process and to corrosion. The slightest reduction inthe thickness of this layer may suffice to jeopardize the safe start ofthe process in the nanostructures for example.

It is therefore very important to precisely control the immersion andwetting procedures. Technically, such a control is not readily possiblebecause of the lack of electrical contact between the electrolyte fluidand the wafer prior to immersion and of the electrolyte dependentequilibrium potential obtained after immersion. Depending on theelectrolyte composition, the starting layer may corrode to a greater orlesser, unforeseeable extent.

It has been found that the parameters that more specifically influencethe metal removal are the partial voltages which, summed, yield thetotal electric voltage (clamp voltage) applied between anode andcathode:

During electrolytic metal deposition, a current flows between anode andcathode. An electric voltage composed of the sum of the partial voltagesmentioned is needed to generate said current flow. The total voltage ismore specifically the result of the sum of anodic and cathodic chargetransfer overvoltage, polarization overvoltages and crystallizationovervoltages and also of concentration overvoltages and of the voltagedrop due to the electrolyte resistance and the voltage drops in theelectrical feed lines.

As a rule, it is not known how the measurable clamp voltage distributesbetween the individual voltages. More specifically, variations in thedistribution cannot be registered as only the clamp voltage of thecurrent source e.g., of the rectifier is known. If, during deposition,one of the resistances mentioned or one of the overvoltages listedvaries or if they fluctuate between various wafers to be processed, itwill, at the best, not be possible to interpret the resulting measurablechange of the clamp voltage. At the worst, this change will not even benoted so that metal will possibly be removed with no possibility to findout about it.

As in the semiconductor technology the process safety and thereproducibility of the methods are of prime importance, means had to befound to register the partial voltages. Variations during the processmust be interpreted and identified in order to permit control andcorrection of the process.

In order to at least detect variations in the voltage drop that are dueto the electric resistance of the electrolyte, reference electrodes areutilized, which are disposed directly on the surface of the anode orcathode. For this purpose, the reference electrodes are to be brought soclose to the surfaces that the potential can be measured directly at therespective one of the surfaces. The reference electrodes can for examplebe brought so close to the respective one of the surfaces that thespacing therefrom is less than 1 mm, for example 0.2 mm. Morespecifically, the reference electrodes may also be arranged for examplein the plane of the anode's or cathode's surface beside the surface, butin immediate proximity thereto, although not directly in front of saidsurface. The reference electrodes thereby need not be brought to contactthe surfaces. Another possibility consists in placing a small containercontaining a conductive electrolyte on the respective surface or inproximity thereto, the reference electrode in said container permittingto detect the potential at the surface.

In a preferred embodiment of the invention, two reference electrodes areprovided: the first of the two reference electrodes is arranged at thesurface of the anode and the second reference electrode at the surfaceof the cathode. As the two reference electrodes are disposed inimmediate proximity to the respective one of the electrodes, theinfluence of the voltage drop due to the electric resistance of theelectrolyte can be detected separately in the form of the electricvoltage between the two reference electrodes. The other voltage dropsmeasured between a respective one of the reference electrodes and theanode or cathode at the surface of which said electrodes are disposedinclude the voltage drops occurring in proximity to the anode or cathodesurface and more specifically the charge transfer, crystallization andconcentration overvoltages.

The various voltage drops in different regions of the electrolytic cellcan thus be detected and, as a result thereof, the influence of theafore mentioned factors (such as the type of the electrolyte solution,the properties of the starting layer and others) can be detectedseparately and analyzed accordingly. Variations due to the influencingvariables mentioned may thus be identified so that appropriateprovisions can be made in the event of such changes.

An advantage of the invention is that existing electroplating plants canbe readily retrofitted with the means of the invention since nosubstantial structural extension is needed.

Any reference electrode can be utilized to measure the variationsmentioned Stable reference electrodes more specifically contain a metalthat is in equilibrium with a hardly soluble salt of said metal and anelectrolyte. Such type reference electrodes are for example electrodesof the second or third order since these electrodes provide a constantreference potential. Reference electrodes of the second order arereference electrodes in which the concentration of thepotential-determining ions is determined by the presence of a hardlysoluble compound the ions of which are the same as thepotential-determining ions. Reference electrodes of the third order, bycontrast, are reference electrodes in which the activity of thepotential-determining ions is determined by the presence of two solidphases. Reference electrodes of the second order are more specificallythe calomel electrode, the silver/silver chloride electrode, themercuric sulfate electrode and the mercuric oxide electrode. Referenceelectrodes of the third order may for example be a zinc rod that is inequilibrium with a solution of calcium ions in the presence of aprecipitate made from zinc and calcium oxalate.

One reference electrode is mounted in proximity to the anode, another inproximity to the wafer. The process is controlled by measuring thevoltage between the anode and the first reference electrode, between thefirst reference electrode and the second reference electrode and betweenthe second reference electrode and the cathode during the electrolyticprocess.

The voltage measured between the first reference electrode and thesecond reference electrode is the result of variations of theelectrolyte resistance which are indicative of the unstable compositionof the electrolyte or of the irregular fluid flow within the processingtank.

Variations of the voltage measured between the first reference electrodeand the anode are additionally indicative of an unstable anode process.With a soluble anode being used, this may be due to the consumption ofthe anode, to a change in the anode film or to a varying anode geometry.With an inert (insoluble) anode, such a change in the measured voltagemay also be indicative of a failure of this anode (the active anodelayer may for example peel off) or of poor redox species, e.g., Fe(II)and Fe(III) compounds, supply to the anode, for example in the event themethod described in DE 100 15 146 C1 is performed.

Variations of the voltage measured between the second referenceelectrode and the cathode are indicative of an unstable cathode processsuch as an altered thickness of the starting layer, for example becausethis layer is attacked by the metal deposition solution or because thelayer has never been thick enough.

To control the process, more specifically to prevent the starting layerfrom corroding, a voltage difference between the starting layer and thenearest reference electrode, the second reference electrode for example,can be applied by a power rectifier prior to immersion. The properchoice of the potential of the starting layer permits to prevent it fromcorroding during immersion and possibly in the wetting phase as well. Inorder to obtain a useful result of the measurement, the respective oneof the reference electrodes must be brought as close as possible to theassociated electrode. However, the workpiece (the cathode for example)and the counter electrode (the anode for example) must thereby beprevented from being unshielded in order for the deposited metal to bedistributed as uniformly as possible.

As stable reference electrodes more specifically contain a metal that isin equilibrium with a hardly soluble salt of said metal and anelectrolyte, there is a risk that the electrolyte of the electrolyticprocess be contaminated by the electrolyte of the reference electrode.Such a contamination has to be prevented by all means. In order toovercome this problem as well, the reference electrodes contact thesurface of the anode or of the cathode through at least one capillary.In that the measurement of the voltages between the reference electrodesand between a respective one of the reference electrodes and the anodeand cathode, respectively, is a high resistance measurement, but a verylittle current flows through the measuring arrangement. As a result, thecapillary can be very thin so that contamination of the electrolyte ofthe electrolytic process through the electrolytes of the referenceelectrodes is minimized.

Another improvement with regard to this problem is achieved in thatelectrolyte fluid of the electrolytic process is supplied to therespective one of the reference electrodes via the capillaries. Theelectrolyte of the reference electrode is thus prevented from enteringby diffusion into the deposition electrolyte.

To electrolytically deposit metal, more specifically copper, onto asemiconductor substrate, customary electroplating arrangements, madefrom platinum for example, can be utilized in which the anode and thesemiconductor substrate are paralleled and oriented horizontally ortilted from horizontal. The anode and the semiconductor substrate canalso be oriented vertically. The two electrodes are located in a tankconfigured to suit the purpose, a cylindrical tank for example, whichaccommodates the electrolyte fluid and the electrodes. Usually, theanode is disposed on the bottom of the cylindrical tank and thesemiconductor substrate in the upper portion thereof. To generate adefined fluid flow, the electrolyte fluid can flow through the tank in acertain manner. The reference electrodes may be housed in separatecontainers communicating with the cylindrical tank via the capillariesmentioned. The capillaries are disposed in the wall of the tank in sucha manner that they are located in immediate proximity to the respectiveone of the electrodes.

Another advantage of the present invention is that the device inaccordance with the invention and the method permit to control thevarious partial processes which have been described as taking place atthe various electrodes, with the possibility of measuring certainvoltages (potentials) concurrently. This provision permits to locateproblems in current transfer.

The invention will be described in closer detail with reference to thefollowing figures in which:

FIG. 1 is a schematic cross section of a structure (trench, via) in adielectric on a semiconductor substrate,

FIG. 2 is a schematic representation of the potential differences in ananode and cathode arrangement with a respective reference electrodebeing provided in proximity to the anode and in proximity to thecathode,

FIG. 3 is a schematic sectional view of a deposition cell,

FIG. 4 is a schematic view of a deposition cell for problem analysis ofcurrent fluctuations during deposition.

FIG. 1 shows the coating thickness distribution of the starting layer 2in proximity to and within a structure 4 in a dielectric layer 3 on asemiconductor substrate. In the present case, the structure 4 is 0.2 μmwide and about 1 μm deep. At the surface of the substrate, the startinglayer 2 is approximately 100 nm thick. The starting layer 2 is muchthinner at the bottom of the structure 4, though. There, it is only 5-25nm thick. In this lower region, there is the risk that the layer 2 isremoved to such an extent during immersion and subsequent wetting of thestarting layer 2 with a metal deposition electrolyte that no metal orbut a very thin layer thereof is left at the bottom of the structure 4.As a result, no metal can be deposited onto this site during thesubsequent electrolytic metal plating process.

FIG. 2 is a schematic illustration of the potential difference betweenthe anode 5 and the cathode 1 in the space containing the electrolyte.Current delivered by a current supply 6 flows between anode 5 andcathode 1. The current supply 6 may be a power rectifier for example.The voltage U delivered by the current supply 6 is measured using avoltmeter 7. The voltage U is also termed clamp voltage.

In proximity to the anode 5 there is disposed a first referenceelectrode 8. Likewise, a second reference electrode 9 is disposed inproximity to the cathode 1.

The potential difference in the space containing the electrolyte betweenthe anode 5 and the cathode 1 is labeled with the numeral 10. Forsimplification sake, the potential difference 10 is divided in onlythree portions 11, 12 and 13, the portions 11 and 13 being generated bythe diffusion and crystallization overvoltages and portion 12 arisingfrom the voltage drop generated by the electric resistance of theelectrolyte.

The voltage drops mentioned can be determined in a simple manner bymeasuring the electric voltages between the anode 5 and the firstreference electrode 8 using the first voltmeter 14, between the firstreference electrode 8 and the second reference electrode 9 using thesecond voltmeter 15 and between the second reference electrode 9 and thecathode 1 using the third voltmeter 16. The sum of the partial voltages11, 12, 13 as measured by the voltmeters 14, 15 and 16 yields the clampvoltage U.

The voltage drop 11 is measured by voltmeter 14, the voltage drop 12 byvoltmeter 15 and the voltage drop 13 by voltmeter 16.

FIG. 3 is a schematic illustration of an arrangement forelectrolytically depositing metal on a semiconductor substrate 1. Thearrangement has a tank 20 and an anode 5 on the bottom of the tank 20and a semiconductor substrate 1 employed as a cathode in the upperportion of the tank 20. The tank 20 is filled with electrolyte fluid 22to level 21. Fluid 22 can for example enter the tank 20 from the bottomand flow through the anode 5. The anode 5 is preferably perforated forthis purpose.

In the wall of tank 20, a first capillary 23 is embedded in proximity tothe anode 5 and a second capillary 24 in proximity to the semiconductorsubstrate 1. Electrolyte fluid can flow in a small volume flow throughsaid capillaries 23, 24 into the reference electrode containers 25 and26 mounted to the sides thereof. This prevents electrolyte fluid, whichmay be contained in said containers 25, 26 and has another formulationthan the deposition fluid 22, from entering tank 20. The containers 25,26 accommodate a first reference electrode 8 and a second referenceelectrode 9 that are connected through electrical lines to voltmeters(not shown).

FIG. 4 is a schematic illustration of a deposition cell. The anodes 5are formed by an inert anode basket which is surrounded by a diaphragm(not shown herein) and holds the metal to be deposited, e.g., in theform of shot or pellets. The anodes 5 are located outside tank 20. Theyare coupled through conduits 29 that are coupled into tank 20 by way ofshields 28. The shields 28 act as virtual anodes. A first referenceelectrode 8 is disposed in proximity to the anodes 5. A second referenceelectrode 9 is likewise disposed in proximity to the cathode 1. Theelectric voltages between the cathode 1 and the second referenceelectrode 9, between the second reference electrode 9 and the firstreference electrode 8 and between the first reference electrode 8 andthe anodes 5 are measured using the voltmeters 16, 15, 14, respectively.The tank 20 is completely filled with electrolyte fluid 22.

A test performed in practical operation showed that the amount of metaldeposited onto the cathode 1 was not sufficient. Concurrently, althoughthe voltage applied was 20 V, a very low current of about 100 mA onlywas measured between the cathode 1 and the anodes 5 as compared to usualdeposition cells (10 A at only 2-3 V). The reasons therefore could havebeen for example:

-   -   1. insufficient electric contact between the cathode 1 and the        electrolyte fluid 22,    -   2. disruption of the conduits 29,    -   3. insuffient electric contact between the anodes 5 and the        electrolyte fluid 22 or    -   4. deficient electrolyte flow through the conduits 29.

In designing the measuring device in accordance with the invention,which is comprised here of the two reference electrodes 8, 9 and thevoltmeters 14, 15, 16, the following results could be obtainedsimultaneously in accordance with the invention so that fault findingwas made possible:

Between the cathode 1 and the second reference electrode 9 a voltage ofabout 0.5 V that was stable over time was measured using the voltmeter16. The voltage between the two reference electrodes 8, 9 as measured byvoltmeter 15 was 1 V and stable over time. By contrast, the voltagebetween the first reference electrode 8 and the anode 5 wasapproximately 18.5 V and varied in time with the overall voltage.

These results permitted to dismiss the afore mentioned reasons 1, 2 and4. The problem could be eliminated by improving the electrical contactestablished at the transition between the anodes 5 and the electrolytefluid 22. It was found out that the diaphragm disposed around the anodebasket was no longer wetted in the electrolyte fluid.

In a further example an electrolytic copper bath 22 was used to deposita copper layer on a semiconductor wafer 1. The wafer 1 was provided witha copper seed layer which was aproximately 100 nm thick. The copper seedlayer was coated with a photoresist layer having vias and trenchesexposing the copper seed layer. The copper bath 22 contained coppersulfate, sulfuric acid and a minor amount of sodium chloride as well asgeneric additive components that are usually used to optimizephysico-mechanical properties. The bath 22 was operated in a depositiontank 20 with a design of the tank as shown in FIG. 4. The anode 5 wasinsoluble, being made from an expanded metal sheet of titanium, whichwas activated with noble metal (e.g. platinum). In order to maintain anominal copper ion concentration in the bath 22 copper pieces weredissolved in a separate container (not shown herein) which was in fluidconnection with the tank 20. In order to promote copper dissolution thebath 22 also contained Fe(II) and Fe(III) compounds. A bath suitable forthis purpose is described in DE 199 15 146 C1 for example.

After the wafer 1 had been brought into contact with the copper bath 22and after a certain idle time thereafter, current was switched on tohave the wafer 1 be metallized. Prior to switching the current on thecopper seed layer risked to be etched by the copper plating bath 22 or,more specifically, by the Fe(III) ion compounds in this bath 22. Forthis reason electroplating was a problem if the copper seed layer atleast partially dissolved prior to first copper deposition.

Electroplating was effected by an electrolytic current flowing betweenthe wafer 1 and the anode 5. Safe initiation of copper deposition couldeasily be determined by measuring the voltage between the wafer 1 andthe reference electrode 9 over time in order to ascertain whether safewetting of the wafer 1 occurred and whether a sufficiently thick copperseed layer was still present at the surface of the wafer 1. If thevoltage was not determined to be in the value range expected, deficientelectroplating was expected.

Furthermore the voltage between the wafer 1 and the reference electrode9 was measured. Practicing this measurement during the entireelectroplating process a defined potential control of the wafer 1 wasachieved. This also guaranteed process safety during the entire copperplating process including the method steps of immersion and wetting ofthe wafer 1. It turned out that seed layer etching could be prevented ifa suitable voltage was applied to the wafer 1. Under these conditionsthe wetting period of the wafer 1 could be optimized, i.e. extended.

At the same time processing was also found to be influenced byimponderable conditions at the anode 5. It turned out that too high aplating rate would lead to a material transport in the plating bath 22being rate determining (reaction of Fe(II) to Fe(III)). This could leadto water electrolysis and hence generation of oxygen gas bubbles at theanode 5. At the same time the anode potential was measured to shift.Under unfavourable conditions this shift was detected by measuring thevoltage between the anode 5 and the reference electrode 8. Therefore byascertaining a voltage which was outside the normal range the processingparameters could be suitably adjusted in order to prevent deficientprocessing.

Hence, it turned out that processing deficiencies, which were caused bydeficient anodic and/or cathodic processes, could be detected, if thevoltages mentioned and the clamp voltage deviated from the normalranges. Only by simultaneously measuring these voltages as well as theclamp voltage between the wafer 1 and the anode 5, the causes for thedeficiencies could be found out.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications andchanges in light thereof as well as combinations of features describedin this application will be suggested to persons skilled in the art andare to be included within the spirit and purview of the describedinvention and within the scope of the appended claims. All publications,patents and patent applications cited herein are hereby incorporated byreference.

Listing of Numerals:

-   1 semiconductor substrate (cathode)-   2 starting layer (seed layer, plating base)-   3 dielectric layer-   4 structure in the dielectric layer 3-   5 anode-   6 current supply (voltage source)-   7 voltmeter for the clamp voltage U-   8 first reference electrode-   9 second reference electrode-   10 potential difference-   11 voltage drop at the anode 5-   12 voltage drop in the electrolyte 22-   13 voltage drop at the cathode 1-   14, 15, 16 voltmeters-   20 tank-   21 fluid level-   22 electrolyte fluid-   23, 24 capillaries-   25, 26 reference electrode containers-   27 electrical lines-   28 shields (virtual anode)-   29 conduit-   U clamp voltage

1. A device for monitoring an electrolytic process, comprising at leastone anode and at least one cathode, at least one reference electrodebeing disposed at the surface of the at least one anode or at thesurface of the at least one cathode, at least one voltmeter beingrespectively provided for detecting the electric voltages between the atleast one anode and the at least one reference electrode and between theat least one reference electrode and the at least one cathode.
 2. Thedevice according to claim 1, wherein at least one first referenceelectrode is disposed at the surface of the at least one anode and atleast one second reference electrode is disposed at the surface of theat least one cathode and wherein a voltmeter is respectively providedfor detecting the electric voltages between the at least one anode andthe at least one first reference electrode, between the at least onefirst and the at least one second reference electrode and between the atleast one second reference electrode and the at least one cathode. 3.The device according to one of the aforementioned claims 1-2, whereinthe at least one reference electrode communicates through capillarieswith the surface of the at least one anode or with the surface of the atleast one cathode.
 4. The device according to claim 3, wherein means areprovided by means of which electrolyte fluid is deliverable through thecapillaries to the at least one reference electrode.
 5. The deviceaccording to one of the aforementioned claims 1-2, wherein the at leastone anode and the at least one cathode are paralleled and orientedhorizontally or tilted from horizontal.
 6. The device according to oneof the aforementioned claims 1-2, wherein the cathode is a wafer or achip carrier substrate and the anode is a metal plate.
 7. A method ofmonitoring an electrolytic process in an electrolytic cell comprised ofat least one anode and of at least one cathode, at least one referenceelectrode being disposed at the surface of the at least one anode or atthe surface of the at least one cathode, at least one voltmeter beingrespectively provided for detecting the electric voltages between the atleast one anode and the at least one reference electrode and between theat least one reference electrode and the at least one cathode, saidmethod involving the following method steps: a) providing an electriccurrent flow between the at least one anode and the at least onecathode, b) concurrently detecting the respective electric voltagesbetween the at least one anode and the at least one reference electrodeand between the at least one reference electrode and the at least onecathode.
 8. The method of claim 7, wherein at least one first referenceelectrode, which is disposed at the surface of the at least one anodeand at least one second reference electrode, which is disposed at thesurface of the at least one cathode are provided, method step b)including the following partial method steps: b1) detecting the electricvoltage between the at least one anode and the at least one firstreference electrode, b2) detecting the electric voltage between the atleast one first reference electrode and the at least one secondreference electrode and b3) detecting the electric voltage between theat least one second reference electrode and the at least one cathode. 9.The method according to one of the claims 7 and 8, wherein the at leastone reference electrode is brought into contact with the surface of theat least one anode or with the surface of the at least one cathode byway of capillaries.
 10. The method according to claim 9, whereinelectrolyte fluid is delivered through the capillaries to the at leastone reference electrode.
 11. The method according to one of the claims7-8, wherein the at least one anode and the at least one cathode areparalleled and oriented horizontally or tilted from horizontal.
 12. Themethod according to one of the claims 7-8, wherein the cathode is awafer or a chip carrier substrate and wherein the anode is a metal plateand wherein the metal is electrolytically deposited on the wafer.